1. Field of the Invention
The present invention relates to battery packs, and, more particularly, to a protection circuit for a battery pack.
2. Description of the Related Art
Recently, compact and portable electronic/electrical appliances, such as cellular phones, lap-top computers, camcorders and the like, have been actively developed and produced, and require battery packs for portable operations. For economic reasons a battery pack typically includes secondary battery that is rechargeable, such as, a nickel-cadmium (Ni—Cd) battery, a nickel-metal hydride (Ni-MH) battery, and an lithium (Li) battery. Specifically, the battery packs having an Li secondary battery are widely used because the Li secondary battery has an operating voltage three times greater than that of the Ni—Cd battery or the Ni-MH battery, and has an energy density per unit weight greater than that of the Ni—Cd battery or the Ni-MH battery.
Typically, the battery pack includes a battery group having one or more rechargeable secondary batteries, an external terminal for connecting the battery group with an external power supply or a load, a first protection circuit for controlling the charging and discharging of the battery group in response to the charged state of the battery group, a second protection circuit for preventing overcurrent from flowing through the battery group, and a thermal fuse electrically connected between the battery group and the external terminal. The secondary battery may include one or more rechargeable bare cells, but the battery group may even be just one bare cell.
The thermal fuse and the second protection circuit electrically disconnect the battery group from the external terminal to prevent the battery pack from starting a fire, burning or exploding when overcurrent flows through the battery group or the internal temperature of the battery pack gets above a predetermined temperature.
However, since the thermal fuse and the second protection circuit are disposable devices, reuse of the battery pack is made impossible when the thermal fuse and the second protection circuit electrically disconnect the battery group from the external terminal. That is, use of the battery pack becomes impossible despite the fact that the battery pack can be continuously used when the internal temperature of the battery pack decreases, or when the internal temperature of the battery pack temporarily increases above the temperature for activating the thermal fuse.